The present invention relates to a method for sealing a cylindrical or truncated cone-like container body containing a hot or cold liquid such as milk, juice or others.
When a hot liquid is poured into a container to be sealed, two problems often occur. One of such problems will now be explained. When the hot liquid contained in the sealed container becomes cooled down to a room temperature, the volume of the liquid itself is reduced. Also, the head air space within the container becomes small in volume. As a result, the container body deforms inwards or partly sinks. This is not good for its appearance and reduces its commercial value. If a glass or metalic container is used, the air pressure in the head space of the container is merely reduced, but the container body itself does not deform, provided that no additional force is applied to the container body. In contrast thereto, the paper container does not have such a structure strength as can endure against the reduction of the air pressure of the head space within the container. Therefore, the container body made of paper deforms when the air pressure of the head space in the container is reduced. The air just adjacent to the top surface of the hot liquid in the container is relatively warm but has a lower temperature than that of the liquid immediately before the cover is sealed to the upper open end of the container body. After the sealing of the container, the air in the container becomes heated by the hot liquid and then expands. However, when the liquid becomes again cooled down to a room temperature, the volume of the head space is apt to become also smaller than that immediately after the sealing of the container. In addition, the volume of the liquid itself becomes reduced because the temperature of the liquid changes from a high temperature to a room temperature. For example, when a juice at the temperature 93.degree. C. is poured into the container and the juice therein becomes cooled down to 20.degree. C. after its sealing, the volume of the juice becomes reduced by about 3%, and the head space becomes reduced by about 20%. As the result, the container body partly deforms inwards to form an unshapely dent on the container body. Such deforming of the container body strikingly makes the commercial value of the goods reduced.
The other problem will be next explained. When the cover is sealed onto the upper open end of the container body, the air of the head space formed within the container becomes heated and expands by the heat of the hot liquid therein. Therefore, immediately after the sealing head is removed from the cover; the air pressure is applied to the sealed portion between the cover and the container body, whereby pinholes or leaks are apt to be formed at the sealed portion. For example, when the cover adheres to the container body of a truncated cone shape into which the juice at the temperature of 93.degree. C. is poured, the air of the head space in the container expands by about 20% or more by the heat of the juice. Since the width of the sealing portion between the cover and the upper open end of the container body is very narrow, the pinholes or leaks for air flows are apt to be formed thereat in particular if the heat-sensitive sealing material such as polyethylene or the like remains softened immediately after its sealing. Such phenomenon often occurs if the temperature of the liquid immediately after its pouring into the container is close to the softening temperature of the resin laminated on the upper end of the container body for the purpose of sealing.